That is so cool, I hought about trying to build something like this but with seat belts for when I did need an extra seat, it would be horribly uncomfortable but with seatbelts it would pass around here, and would come in handy when needed.

Especially if you held an inflated beach ball in your lap to serve as an airbag in case of rapid deceleration....

 

I know an older lady that says she likes to take the old man for drives.  That way the car is then equipped with a passenger-side "airbag."